Methods and apparatus for producing coherent or monolithic elements

ABSTRACT

Methods and apparatus for producing coherent or monolithic elements, such as mono-crystalline semi-conductor rods, preferably by growing or drawing one or more such elements upwardly from an off-center portion of a molten pool in the top of a cake of material while rotating the cake and/or rod or rods, for example, in the same direction and at the same rate; heat balancing the element as it is grown from the pool and maintaining the rod-pool location positioned relative to heating means, as by adding additional materials to the pool as the element is withdrawn; and/or providing relative feeding movement between the cake and rod; and providing a chilled mold beneath the cake.

REFERENCE TO RELATED APPLICATION

This is a continuation of co-pending application Ser. No. 898,136 filedon Aug. 19, 1986, abandoned, which was a divisional of Application Ser.No. 564,773 filed Dec. 27, 1983, Now U.S. Pat. No. 4,650,540 which, inturn, was a continuation of Application Ser. No. 594,257 filed July 9,1975, now abandoned, which, in turn, was a continuation-in-part ofApplication Ser. No. 614,728 filed Feb. 8, 1967, now abandoned.

BACKGROUND AND SUMMARY OF THE INVENTION

This invention relates to methods and apparatus for producing coherentor monolithic elements, such as rods of amorphous or monocrystallinematerial, and, more particularly, to methods of growing such elementswhich are characterized by high purity and freedom from dislocations anddiscontinuities.

The most important present commercial utilization of the presentinvention is in the production of semi-conductor materials, such as areextensively utilized in the electronics industry, but it is to beunderstood at the outset that the present invention may be utilized forthe production of other than semi-conductor crystals or materials, suchas crystalline materials in the nature of rubies for laser applications,grainless metals, amorphous materials, and the like. For purposes ofillustrating the present invention, however, the material produced willbe assumed to be monocrystalline semi-conductor materials having uniformphysical and chemical properties.

The production of crystals having exceedingly high purity or freedomfrom dislocations has become increasingly important, especially for theproduction of semi-conductor material for use in the electronicsindustry as well as for use in basic research.

Monocrystalline materials, e.g., semi-conductor materials comprising,e.g., silicon or germanium have previously been produced by severalbasic methods. All of the previously suggested techniques have beenbased upon variations of two basic crystal growth mechanisms. One suchmechanism is the Czochralski technique and the other is that of zonemelting. The Czochralski technique, and the various variations thereof,produce monocrystalline structures by pulling a single crystal from amolten mass of raw material contained within a crucible. Zone meltingtechniques do not use a crucible, but rather melt only a very smallregion of a polycrystalline bar, as by radio-frequency induction, anddepend upon surface tension of the molten zone to retain the moltenmaterial relative to the crystalline material.

The crucibles utilized in the Czochralski growth techniques aregenerally fabricated of a relatively inert material, such as quartz. Thepresence of even an inert material such as quartz in contact with themolten material invariably produces some oxygen contamination of themelt at the relatively high temperatures necessary. The zone meltingtechniques, on the other hand, depend upon surface tension of the moltenzone to retain the molten material in position and generally produceexcessive crystal dislocations, strains or discontinuities because ofthe large temperature differentials inherently present in such a thinmolten zone.

Accordingly, it is a primary object of the present invention to producemonolithic structures or elements which are of high purity, coherent,and dislocation or discontinuity-free, and possessing uniform physical,chemical and electrical properties.

Another primary object of the present invention, in addition to theforegoing object, is to produce high purity monocrystalline structureswhich are relatively dislocation-free.

Another primary object of the present invention, in addition to theforegoing objects, is to produce monocrystalline structures of highpurity.

Another primary object of the present invention, in addition to each ofthe foregoing objects, is to produce such structures of larger diameterthan heretofore capable of production.

Another primary object of the present invention, in addition to each ofthe foregoing objects, is to produce such structures by controlledaccretion from a molten mass.

Yet another primary object of the present invention, in addition to theforegoing objects, is to provide methods and apparatus for producingsuch structures continuously, rather than by batch techniques.

A still further primary object of the present invention, in addition tothe foregoing objects, is to provide methods and apparatus for producingsuch structures which enable finely controlled doping thereof to beeasily and readily accomplished.

A still further primary object of the present invention, in addition toeach of the foregoing objects, is to provide such methods and apparatusproducing a generally uniform physical and chemical property profileacross the growing structure-melt interface.

Another and yet still further primary object of the present invention,in addition to the foregoing objects, is to provide such methods andapparatus which enable relatively uniform relative movement between thegrowing structure and the melt.

Another and yet still further primary object of the present invention,in addition to the foregoing objects, is to provide such methods andapparatus which precludes contamination of the structure material.

It is also a primary object of the present invention, in addition to theforegoing objects, to provide such methods and apparatus having aminimum complexity and a maximum ease of operation.

It is a feature of the present invention that the structure productionmay be terminated at any desired time and the melt allowed to solidify,without damage or loss of materials.

The invention resides in the combination, construction, arrangement anddisposition of the various component parts and elements incorporated inimproved methods and apparatus for element growing in accordance withthe principles of this invention. The present invention will be betterunderstood and objects and important features other than thosespecifically enumerated above will become apparaent when considerationis given to the following details and description, which when taken inconjunction with the annexed drawing describes, discloses, illustratesand shows certain preferred embodiments or modifications of the presentinvention and what is presently considered and believed to be the bestmode of practicing the principles thereof. Other embodiments ormodifications may be suggested to those having the benefit of theteaching herein, and such other embodiments or modifications areintended to be reserved especially as they fall within the scope andspirit of the subjoined claims.

IN THE DRAWINGS

FIG. 1 is a cross-sectional elevational view of monolithic structuregrowing apparatus constructed in accordance with the principles of thepresent invention;

FIG. 2 is a plan view taken along line 2--2 of FIG. 1;

FIG. 3 is a partial cross-sectional elevational view of anotherembodiment of apparatus in accordance with the principles of the presentinvention;

FIG. 4 is a partial cross-sectional elevational view schematicallyillustrating one form of temperature profile produced by priorapparatus;

FIG. 5 is a view similar to FIG. 4 schematically illustrating anothertemperature profile produced in previous apparatus;

FIG. 6 is a view similar to FIGS. 4 and 5 schematically illustrating theideal temperature profile produced by the present invention;

FIG. 7 is a schematic view showing diagrammatically the relativemovement between the rod and cake or pool of the apparatus in accordancewith the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawing, and particularly to FIGS. 1 and 2thereof, there is shown and illustrated a preferred form of apparatusfor producing monolithic or coherent structures or elements, such asmonocrystalline rods designated generally by the reference character 10,which is constructed in accordance with the principles of the presentinvention.

The apparatus 10 comprises a furnace enclosure 12 adapted to containherewithin an inert atmosphere or a vacuum, in accordance withconventional practice. The furnace enclosure 12 may, for example,comprise a cylinder 14, a cap 15 and a base 16 adapted to be securedremovably together by means, such as a plurality of clamps 18. Thecylinder 14 is provided with means, such as generally radially extendingshoulders 20 adapted to be engaged by the clamps 18 and sealing means,such as annular seals 22, is adapted to be clamped between the shoulders20, the lower surface of the cap 15 and the upper surface of the base16, respectively, to provide within the furnace enclosure 12 a sealedchamber 24. Conduit means, such as passages 26, is provided for enablingevacuation or atmospheric control of the chamber 24.

The furnace enclosure 12 is provided with cooling means, such as acooling coil 28, adapted to enable circulation therethrough of a coolingmedium, such as water, in contiguous relationship to the furnaceenclosure 12. The lower portion of the cylinder 14 is provided on theinterior surface thereof with insulation means, such as an annular layerof insulation 30 so that the chamber 24 will be thereby subdivided intoa heating portion 32 and a cooling portion 34. Rather than aconventional insulation, the insulation 30 may also comprise a radiationshield or the like, since at the temperatures involved, most of theenergy is in the form of radiation, and furthermore, when working underevacuated or vacuum conditions, radiation shielding or reflecting meansare all that is required to provide the desired insulation. A cake ofmaterial 36, which may be polycrystalline, is rotatably supported withinthe heating portion 32 of the chamber 24, as on a support platform ordisk 38 fabricated of a relatively rigid material which is structurallyassociated with a shaft 40 extending generally centrally through thebase 16 whereat the shaft 40 is rotatably supported by sealing bearingmeans, illustrated schematically as stacked chevrons 42. A drive 44,which preferably is capable of providing both rotational and vertical oraxial movement of the shaft 40, is structurally associated at the lowerend of the shaft 40 to at least rotatably drive the shaft 40, disk 38and cake of material 36 and, preferably, to enable advancement and/oraxial positioning thereof.

Primary heating means, such as a resistance heating element or aradio-frequency induction heating element 46, is provided positionedadjacent the upper surface of the cake 36 to provide a pool 50 of moltenmaterial within the top surface of the cake 36. The primary heatingmeans 46 is so constituted and arranged as to provide the pool of moltenmaterial 50 in such a manner as to enable the pool 50 to be contained bythe cake 36, that is, to enable an edge wall 52 of generally annularconfiguration to remain in the solid state. Hence, the pool of moltenmaterial 50 contacts only the cake 36, and, of course, the atmospherewithin the chamber 24. Hence, contamination of the pool of moltenmaterial 50 from contact with a crucible, or the like, is entirelyprecluded in the present apparatus. Moreover, the annular edge wall 52enables the pool of molten material 50 to be of a substantial depth, sothat the temperature profile thereof adjacent the surface may besubstantially constant. Since the pool of molten material 50 iscontained by the cake 36, rather than by means of a separate crucible,as in conventional Czochralski apparatus, or by surface tension as inconventional float zone apparatus, the depth of the pool of moltenmaterial 50 may be substantial while precluding loss of molten materialfrom the pool or contact by the molten material with contaminants, suchas an external crucible.

The configuration and power input to the heating means 46 may be soselected as to configure the pool of molten material 50 to generallyannular configuration, as shown, or may be sufficient to melt asufficient amount of material to form the pool of molten material 50across the center of the cake 36, as shown by the line 54.

A rod of monocrystalline material 56 may be pulled or grown upwardlyfrom the melt or pool of molten material 50 as is well known in the art.However, rathan than being pulled from a location generally central ofthe melt or pool of molten material 50, the rod of monocrystallinematerial 56 is preferably pulled from an off-center location, as shown.In accordance with conventional practice, a seed crystal 58 is securedwith a pull rod 60, as by means of a chuck 62 fabricated of molybdenum,or the like. The pull rod 60 is rotated by means, not shown, and slowlymoved axially upwardly from the melt or pool of molten material 50 as byfeed means 64.

The pull rod 60 preferably passed upwardly through a main port 66provided in the cap 15, with the feed means 64 being disposed outwardlyof the furnace enclosure 12, as shown. The feed means 64 is showndiagrammatically in simplified form as clearly comprising a plurality offeed wheels, or the like, but in actual practice, since the rate of pullmust be exceedingly slow and at a finely controlled rate, the feed means64 in actuality preferably comprises a very accurate slow speed drive,such as a worm driven frame, or the like.

The main port 66 comprises an aperture 70 extending through the top wallor cap 15 and provided with sealing means. Preferably, the sealing means72, which, for simplicity of illustration, is shown as an O-ring seal,comprises a more sophisticated seal, such as stacked chevron seals. Thesealing means 72 preferably is also capable of maintaining a sealingrelationship with the pull rod 60 and with the rod of monocrystallinematerial 56, regardless of whether the atmosphere within the furnaceenclosure 12 is above or below the external pressure. Moreover, thesealing means 72 should be capable of providing centering support to thepull rod 60 and to the rod of monocrystalline material 56. Inconventional or heretofore known apparatus of this general type, the rodof monocrystalline material has been produced by a batch technique, andonly the pull rod has been reciprocated through such a seal. The presentinvention, as herein pointed out, is capable of continuous production ofthe rod of monocrystalline material 56. Hence, since the rod ofmonocrystalline material produced is not, generally, or uniform diameterand generally does not comprise a smooth surface, the sealing means 72is preferably capable of maintaining a proper seal against such anuneven surface. To further aid in providing a proper seal through thecap 15, and to provide additional support to the pull rod 60 and the rodof monocrystalline material 56, which is necessary to prevent a whippingaction thereof due to the rotation, the main port 66 may furthercomprise an auxiliary bell structure 74 provided with an aperture 76 isspaced apart aligned relationship to the aperture 70. The aperture 76,similar to the aperture 70 is provided with sealing means 78 similar indesign and function to the sealing means 72. Accordingly, the main port66 is thereby provided with a buffer chamber 80 adapted to be evacuatedor pressurized in accordance with the conditions existing within thechamber 34, as by means of conduit means 82 to thereby precludecontamination of the chamber 34. Preferably, the conditions imposed atthe buffer chamber 80 are such as to provide a small inert gas flow pastthe sealing means 72 into the furnace enclosure 12 to preclude anycondensation or fouling from occurring at the region of the sealingmeans 72. Without such small flow, the high temperature present at thepool of molten material 50, and the vapor pressure of the moltenmaterial would ordinarily produce a condensation of the material,similar to that occurring in conventional vacuum deposition techniques.

As the rod of material 56 is pulled or grown from the melt or pool ofmolten material 50, the cake 36 may be advanced or raised to retain thesurface of the melt or pool of molten material 50 appropriately spacedfrom the main heating means 46. However, it is within the ambit of thepresent invention to enable the continuous production of monocrystallinerod material. Accordingly, the cake 36 need only be initially verticallypositioned and auxiliary port means 84 may be provided structurallyassociated with the top wall or cap 15 of the bell jar 14 to enablecontinuous introduction of a rod of additional material 86 into thechamber 34 and into the melt or pool of molten material 50, in order tomaintain the level of the melt. Furthermore, the auxiliary port means 84may be utilized to enable the introduction of rods of doping material 87(see FIG. 2) into the melt or pool of molten material 50. The auxiliaryport means 84, similarly to the main port 66 comprises a bell jarstructure 88 defining a buffer chamber 90 adjacent the cap 15 of thefurnace enclosure 12. The cap 15 of the furnace enclosure 12 and theauxiliary bell jar structure 88 are each provided with axially alignedapertures 92 and 94, respectively, the number thereof being dependentupon the number of rods 86 and 87 to be introduced. The auxiliary portmeans 84 may further be provided with means to seal off the apertures,if less than the maximum number of rods are to be inserted. Theapertures 92 and 94 are each provided with sealing means 96 and 98,respectively, similar to the seals 72 and 78. The buffer chamber 90 isconnected with the conduit means 82 to enable evacuation or inert gaspressurization of the buffer chamber 90 to preclude the introduction ofcontaminants into the main chamber 34. Auxiliary drive means 99, whichare shown only schematically, may be provided to feed the conduit means82 to enable evacuation or inert gas pressurization of the bufferchamber 90 to preclude the introduction of contaminants into the mainchamber 34. Auxiliary drive means 99, which are shown onlyschematically, may be provided to feed the rods 86 and 87. The drivemeans 99 may be automatically actuated, or may be manually controlled.Furthermore, the dopant may be added to the melt as a constituent of thepolycrystalline material which comprises the cake 36 or the rod 86.

Auxiliary cake heating means, such as resistance or radio-frequencyinduction heating coils 100 are provided adjacent the cake 36 tomaintain the temperature of the cake 36 close to but just below themelting temperature of the cake 36. The cake heating means 100 may alsocomprise a susceptor, or the like, if required for effective heating ofthe cake 36. Furthermore, auxiliary heating means, such as resistanceheating means or radio-frequency induction heating means 102 may beprovided to pre-heat the rod of polycrystalline material 86 or the rodsof dopant material 87.

The cooling of the rod of monocrystalline material 56 preferably isrigidly controlled. To this end, a cooling sleeve 104 provided withcoolant passages 106 is positioned within the furnace enclosure 12 andis so configured and arranged that the rod of monocrystalline material56 will be drawn therethrough. The cooling sleeve 104 may cool the pullrod 60 and rod of monocrystalline material 56 by convection andradiation or may be constructed and arranged to cool them directly byconduction. The cooling sleeve 104 may further be provided with bearingmeans 108 to aid in maintaining the pull rod 60 and the rod ofmonocrystalline material 56 in the correct alignment thereof. Thecooling sleeve 104 may be in a fixed position relative to the cake 36 ormay be vertically movable so that the distance between the coolingsleeve 104 and the surface of the melt or pool of molten material 50 maybe varied to control the cooling rate of the rod of monocrystallinematerial 56. Supplementary heat control means 109, which preferably, iscapable of selectively heating or cooling the rod of monocrystallinematerial 56 as it is being drawn from the pool of molten material 50 maybe provided. The supplementary heat control means 109 may, for example,comprise a coil through which an appropriate heat transfer medium may becirculated. The supplementary heat control means 109, together with thecooling sleeve 104 and main heating means 46 therefore enables thetemperature profile across the rod melt interface to be readilycontrolled.

A chilled mold 110 may be positioned below the cake 36 and provided withcooled passages 112. The chilled mold 110 preferably is fabricated of amaterial having high heat conductivity, such as copper, or the like.With the chilled mold 110 positioned beneath the cake 36, if the cake 36is overheated or, for any other reason, molten material from the cakefalls thereon, the molten material will immediately solidify by contactwith the chilled mold in such a short period of time that contaminationof the material will not take place.

Since only a small portion of the cake 36 is utilized to form the meltor pool of molten material, it is not necessary that the entire cake 36be composed of pure material. The cake 36 may comprise relatively impurematerial which is previously machined or otherwise provided with arecess 114 therein. A relatively thin layer of pure polycrystallinematerial 116 may then be cast into the recess 114 to form a buffer layerbetween the melt or pool of molten material and the remainder of thecake 36. In this way, only a relatively small amount of pure material,which is quite expensive, is required and the predominant portion of thecake 36 may be fabricated or relatively inexpensive material.Furthermore, the remainder of the cake 36 may even comprise a materialsubstantially dissimilar to that of the pool of molten material 50, suchas a conventional crucible material. Since the buffer layer 114precludes contact of the molten material 50 with the remainder of thecake 36, contamination of the melt or pool of molten material iseffectively precluded.

Preferably, however, the cake 36 does comprise a material which issubstantially similar to the material being drawn, or grown, at leastinsofar as temperature-density characteristics are concerned. If thecake 36 does comprise a material having similar temperature-densitycharacteristics throughout, then the growing operation may be terminatedat any desired time and the melt be permitted to cool and solidifywithout resulting in any damage. In conventional crucible melting of thematerial, the molten mass must be entirely removed from the cruciblewithout permitting the melt to solidify within the crucible, since thecrucible would be destroyed upon solidification of the melt.

Cut-off means 118, such as an abrasive wheel, or the like, may also beprovided to enable the rod of monocrystalline material 56 to be severedas it emerges from the furnace enclosure 12.

As hereinbefore pointed out, both the cake 36 and the rod ofmonocrystalline material 56 are rotated during the drawing, growing orpulling process. Preferably, the rod of monocrystalline material 56 andthe cake of polycrystalline material 36 are rotated in the samedirection and at the same rate of rotation. Also, if the rod ofmonocrystalline material 56 is entirely offset from the axis of rotationof the cake 36, as shown in FIGS. 1 and 2, the relative velocity at themelt-crystal interface will be constant in the tangential directionenabling optimum production of dislocation-free crystals. Furthermore,such arrangement, especially in conjunction with the presence of thecooling sleeve 104 and supplementary heat control means 109, enables anoptimum temperature profile at the melt-crystal interface.

When the rod 56 and cake 36 are rotated in the same direction and at thesame rate of rotation, and the rod 56 is entirely offset from the axisof rotation of the cake 36, the geometric relationship between the rodand cake is such that the relative motion at the cake-rod interface issubstantially uniform across the entire face of the rod. This uniquefeature of the present invention contributes to the production ofdislocation or discontinuity-free crystals having uniform physical andchemical properties. Referring to FIG. 7, it will be readily seen thatthe translation past the rod 56 is substantially uniform under theseconditions. Where μ is any unit of length, the translation of the rod 56at its outer surface per revolution will be πd or 2 μπ. The translationof the cake per revolution will be as follows:

(1) past center of rod--6 μπ

(2) past outer edge of rod--8 μπ

(3) past inner edge of rod--4 μπ

Accordingly, the relative movement per revolution of the cake past therod will be as follows:

(1) at center of rod--6 μπ

(2) at outer edge of rod--8 μπ-2 μπ=6 μπ

(3) at inner edge of rod--4 μπ+2 μπ=6 μπ

It will be readily seen, therefore, that the relatively movement acrossthe entire face of the rod is the same, namely, 6 μπ per revolution.

Preferably, one or more observation windows 111 are provided in thefurnace enclosure 12, as shown in FIG. 1, positioned to enable visualinspection of the enterior thereof, and especially the rod meltinterface region. The windows 111 may, for example, be sealingly securedin viewing ports 113 extending generally angularly outwardly of thecylinder 14. Gas inlet means 115 is also preferably provided at eachviewing port 113 adjacent the window 111 thereof to keep the windowclear from condensation or deposition materials volatized from the poolof molten material 50.

While the monocrystalline rod 56 preferably is pulled from an off-centerlocation, as shown in FIGS. 1 and 2, it is within the scope of thepresent invention to pull the rod of monocrystalline material from thecenter of the melt, as in conventional Czochralski techniques, butwithout requiring the use of a crucible. For example, with reference nowto FIG. 3 of the drawings, there is schematically shown and illustrateda cake of polycrystalline material 120 supported within a furnaceenclosure 122. The cake 120 is mounted, for example, upon a quartz disk124 structurally associated with a rotatable and advanceable supportshaft 126. The shaft 126 is structurally associated with the furnaceenclosure 122 as by means of a sealed bearing 128. Feed means, such asfeed rollers 130 are provided for advancing the shaft 126 and theassociated quartz disk 124 and cake 120. Heating means, such as aresistance heating element or radio-frequency induction heating element132, is provided adjacent the upper surface of the cake 120 to maintaina portion 134 thereof in the molten state. The power input to theheating means 132 should be such that the molten portion 134 should notextend to the periphery of the cake 120 but should be contained by asolid peripheral portion 136 of the cake 120. A rod of monocrystallinematerial 138 may be drawn or pulled from the pool of liquid material orportion 134 in the same manner as such a rod would be pulled from a meltcontained within a crucible according to Czochralski techniques.

As the rod of monocrystalline material 138 is drawn or pulled from thepool of liquid material or portion 134, the element or induction heatingelement 140 may be provided adjacent the rim portion 136 of the cake 120to cause the rim or peripheral portion 136 to melt upon relativemovement therebetween to preclude formation of a deep shell.

All of the other features of the apparatus shown in FIG. 1, such as thecooling sleeve 104 or the chilled mold 110 may, of course, be providedwith the apparatus of FIG. 3.

With reference now to FIGS. 4, 5 and 6 of the drawings, there isrespectively shown and illustrated lines 142 which are indicative of thetemperaure profile present across the rod, melt, and rod-melt interfaceunder varying conditions. If the temperature profiles shown in FIGS. 4and 5 are present, then the central portion of the crystal rod will becooling more slowly or more rapidly than the exterior portions thereof,respectively. Such irregular cooling or solidifying of the crystal mayresult in a non-linear crystallization front and, accordingly,non-uniform transverse physical, chemical and electrical properties. Inthe present invention, when the rod 56 is grown from an off-centerlocation, a better temperature profile is obtainable and a bettercrystallization front is attained, especially when the rod is properlycooled or heat balanced, which in the present invention may becontrolled by the positioning of the cooling sleeve 104, the positioningof the supplementary heat control means 109, and the energy conditionsthereof. The ideal temperature profile, as shown in FIG. 6, which willresult in a minimum of crystal dislocation, may be accomplished by thepresent invention.

While the invention has been shown, illustrated, described and disclosedin terms of certain preferred embodiments or modifications which it hasassumed in practice, the scope of the invention should not be deemed tobe limited by the precise embodiments or modifications herein shown,illustrated, described or disclosed, such other embodiments ormodifications as may be suggested to those having the benefit of theteachings herein being intended to be reserved especially as they fallwithin the scope and spirit of the claims hereto appended. Particularly,while the invention has been shown, described, illustrated and disclosedin terms of the production of monocrystalline rods, especially for usein the production of semi-conductor devices, it is to be particularlyunderstood that the principles of the present invention are likewisesuitable for the growing of other materials, whether monocrystalline,polycrystalline, amorphous, metals or the like.

What is claimed is:
 1. Method of forming crystalline material into largediameter substantially mono-crystalline rods having a minimum ofdislocations comprising, at least the steps of, rotating a substantiallysolid cake of crystalline material within a controlled atmosphere abouta generally vertical central axis; heating at least a portion of theupper surface of such cake on at least one side of the rotational axisto a temperature just above the melting point of the material so thatthe heat and rotation provide a pool of molten material encircling therotational axis supported and contained by the cake; slowly withdrawinga substantially mono-crystalline rod upwardly from the surface of saidpool so that material from said pool may crystallize at the interfacetherebetween to elongate the crystal structure of the rod; and rotatingthe rod at substantially the same rotational velocity and in the samedirection about a generally vertically disposed axis parallel to andoff-center from the rotational axis of the cake so that the relativemotion at the pool-rod interface is substantially uniform across theentire face of the rod and thermal asymmetrics are minimized.
 2. Methoddefined in claim 1 further comprising heating the entire cake to atemperature just below the melting point thereof so that the temperaurethroughout the pool will be substantially uniform.
 3. Method defined inclaim 1 further comprising cooling the rod at a uniform rate from afixed location relative to the surface of the pool so that thetemperature profile at the interface will be substantially flat anduniform to reduce the incidence of thermal stresses and crystaldislocations.
 4. Method defined in claim 2 further comprising providinga uniform temperature condition to the rod at a fixed distance above thesurface of the pool so that a uniform and controlled heat balance willbe produced and the temperature profile at the rod-pool interface willbe substantially uniform, flat and constant.
 5. Method defined in claim4 wherein said step of providing a uniform temperature conditioncomprises passing the rod through a cooling sleeve having a controlledreduced temperature for uniformly cooling the rod.
 6. Method defined inclaim 4, wherein said step of providing a uniform temperature conditioncomprises passing the rod through a heating sleeve having a controlledelevated temperature for uniformly heating the rod.
 7. Method defined inclaim 4 wherein said step of providing a uniform temperature conditioncomprises passing the rod through heating and cooling zones to controlthe growing profile across the rod at the rod-pool interface.
 8. In amethod of forming crystalline materials into substantiallymono-crystalline rods wherein a pool of molten crystalline material isrotated about a generally vertical axis, and a substantiallymonocrystalline rod is withdrawn upwardly from the surface of the poolso that material from the pool crystallizes at the interfacetherebetween to elongate the crystal structure of the rod, theimprovement comprises:providing a controlled temperature zone at theinterface between the pool and the rod; and controlling the temperatureof the controlled temperature zone so that as the rod is passed throughthe controlled temperaure zone, the horizontal cross sectional growingprofile across the rod at the rod-pool interface is controlled tomaintain a uniform temperature across said horizontal cross sectionalgrowing profile.
 9. Method defined in claim 8 wherein saidmonocrystalline rod is rotated.
 10. In a method of forming crystallinematerials into substantially monocrystalline rods wherein said rod iswithdrawn from a molten pool so that the material from the poolcrystallizes at the interface therebetween to elongate the crystalstructure of the rod, the improvement comprises:providing a controlledtemperature zone at the interface between the pool and the rod; andcontrolling the temperature of the controlled temperature zone so thatas the rod is passed through the controlled temperature zone at a fixeddistance from the pool, the temperature profile horizontally across thecross-section of the rod at the rod-pool interface is uniformlycontrolled to maintain a uniform temperature across said horizontalcross section of the rod.
 11. Method defined in claim 10 wherein dynamicheat flow in the rod is controlled from the pool-rod interface to adistance beyond which there is no further effect on the growth at theinterface.
 12. Method defined in claim 10 wherein controlled temperaturecondition comprises at least one heat exchange zone.
 13. Method definedin claim 10 wherein temperature profile across rod is flat.
 14. Methoddefined in claim 1 including a vessel containing the pool of moltenmaterial and further comprising disposing a chilled mold below thevessel containing the pool to catch any material that will fall thereonwithout contamination of the material.
 15. Method defined in claim 14wherein said chilled mold is contoured so that solidification ofmaterial cannot damage said mold by expansion or contraction ofmaterial.
 16. Method of forming crystalline material into large diametersubstantially mono-crystalline rods having a minimum of dislocationscomprising, at least the steps of, rotating a substantially solid cakeor crystalline material within a controlled atmosphere about a generallyvertical central axis; heating such cake to maintain the temperaturethereof just below the melting point of the materia; heating at least aportion of the upper surface of such cake on at least one side of therotational axis to a temperature just above the melting point of thematerial so that the heat and rotation provide a pool of molten materialhaving a generally uniform temperature supported and contained by thecake; slowly withdrawing a substantially monocrystalline rod upwardlyfrom the surface of said pool so that material from said pool maycrystallize at the interface therebetween to elongate the crystalstructure of the rod; and providing a uniform and controlled temperaturezone to the rod at a fixed distance from the surface of the pool so thata flat and uniform temperaure will exist horizontally across the crosssection of the interface.
 17. Method defined in claim 16 wherein saidstep of providing a uniform temperature condition comprises passing therod through a cooling sleeve having a controlled reduced temperature foruniformly cooling the rod.
 18. Method defined in claim 16 wherein saidstep of providing a uniform temperature condition comprises passing therod through a heating sleeve having a controlled elevated temperaturefor uniformly heating the rod.